Dual hemispherical collectors

ABSTRACT

A system and method for collecting radiation, which may be used in a lithography illumination system. The system comprises a first surface shaped to reflect radiation in a first hemisphere of a source to illuminate in a second hemisphere of the source; and a second surface shaped to reflect radiation in the second hemisphere of the source to an output plane.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of and claims priority to U.S. patent application Ser. No. 10/394,412, filed Mar. 20, 2003, and International Patent Application No. PCT/US03/40315, filed Dec. 15, 2003. The disclosures of the prior applications are considered part of (and are incorporated by reference in) the disclosure of this application.

BACKGROUND

A microchip manufacturing process may deposit various material layers on a wafer and a photosensitive film or photoresist on the deposited layers. The process may use lithography to transmit light through transmissive optics or reflect light from reflective optics and a reticle or patterned mask onto the photoresist, which transfers a patterned image onto the photoresist. A process may remove portions of the photoresist that are exposed to light. A process may etch portions of the wafer which are not protected by the remaining photoresist. Some of these actions may be repeated.

Extreme ultraviolet (EUV) is one form of lithography. A EUV lithography tool may be used to print a pattern on a photoresist with dimensions that are smaller than dimensions achieved by other lithography tools.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of an Extreme Ultraviolet (EUV) lithography tool.

FIG. 2 is a cross-sectional side view of an apparatus which may be used in a Dense Plasma Focus (DPF) electric discharge source.

FIG. 3 is a cross-sectional view of a dual hemispherical collector system, which may be used in a lithography illumination system.

FIG. 4 illustrates a table of values associated with FIG. 3.

FIG. 5 is a cross-sectional view of another embodiment of a dual hemispherical collector system, which may be used in a lithography illumination system.

FIG. 6 illustrates an example of a hyperbola and ellipse for a shape of the second collector in FIG. 5.

FIG. 7 illustrates a method of using a dual hemispherical collector system.

DETAILED DESCRIPTION

Dual hemispherical collectors are described herein with lithography systems, but the dual hemispherical collectors may be used with other light sources for car headlights, movie/video/slide projectors and other applications outside of lithography.

EUV Lithography

FIG. 1 illustrates one embodiment of an Extreme Ultraviolet (EUV) lithography tool 100, which may also be called a “lithographic exposure system” or a “EUV scanner.” The lithography tool 100 may include a laser 102, a laser produced plasma (LPP) source 104, a plurality of condenser optics 106, a reflective reticle 107 with a pattern, and a plurality of reflective reduction optics 108.

Other embodiments of the EUV lithography tool 100 may include other components instead of or in addition to the components shown in FIG. 1. For example, instead of a laser produced plasma source 104, the lithography tool 100 may have an electric discharge EUV source 200, as shown in FIG. 2.

The EUV lithography tool 100 may create plasma by using a laser-produced plasma (LPP) source 104 or an electric discharge source 200. A laser-produced plasma (LPP) source 104 produces plasma by focusing a laser 102 onto a gas, liquid, or filament jet of a material or materials, such as Xenon.

An electric discharge source produces plasma by pulsing a current discharge (kAs) (like a powerful arc welder) through a gas, such as Xenon. The plasma emits visible and EUV radiation. Excitation of the Xenon molecules causes the electrons to transition between their shells to produce EUV photon radiation.

The EUV source 104 may produce radiation (photons) with a very short wavelength, such as about 13 nanometers. In other embodiments, the photons may have other wavelengths.

As an example, it may be desirable for the EUV lithography tool 100 to deliver about 50-120 watts of “clean” power to projection optics for a throughput of 80 wafers per hour.

Single Hemisphere Radiation Collection

FIG. 2 illustrates an example of radiation collectors 232 in a single hemisphere. FIG. 2 is a cross-sectional side view of an apparatus 200 which may be used in a Dense Plasma Focus (DPF) electric discharge source. The apparatus 200 includes an anode 208, cathode 202 and insulator 212. The apparatus 200 may be used with a buffer gas 206, such as Helium, a source gas 214, such as Xenon, a foil trap 230, a grazing incidence collector 232 and a pump 234. The anode 208 may be coupled to a high voltage source 216, and the cathode 202 may be grounded.

Extreme ultraviolet (EUV) light sources, particularly laser produced plasma EUV sources, may have a finite power output that is radiated into a large solid angle of up to about 4Π. Laser plasma sources tend to be very small, e.g., about 300 microns in diameter. Due to their small size, a laser plasma source may be modeled as a quasi-point source.

The ability to collect the large solid angle light has been limited by several factors, including mirror fabrication, coating requirements, and the large solid angle itself. Currently, near-normal incidence collectors or grazing incidence collectors try to resolve these challenges to collect light. But the collectors are only in a single hemisphere, as shown in FIGS. 1 and 2.

Dual Hemisphere Radiation Collection

The above challenges may be addressed by using collectors in both hemispheres (e.g., 320 and 330 in FIG. 3) around the radiation source/object (e.g., 306), as described with reference to FIGS. 3-7. Dual hemisphere radiation collectors may be implemented with a laser produced plasma (LPP) source 104 of FIG. 1 or any other light source that radiates light into a large solid angle up to about 4Π.

FIG. 3 is a cross-sectional view of a dual hemispherical collector system 300, which may be used in a lithography illumination system, such as the system 100 of FIG. 1. A radiation source 306 in FIG. 3 emits forward radiation 312 in one hemisphere 320 and backward radiation 310 in another hemisphere 330. The backward radiation 310 may be reflected by a first collector (C1) 309 (also called a “surface,” “mirror” or “condenser”) at near normal incidence, i.e., almost perpendicular, e.g., 75-89 degrees. The first collector 309 may have a spherical or aspherical shape, i.e., elliptic or other conics.

A second collector 302 may have a grazing incidence, e.g., less then about 60 degrees, and may reflect forward directed radiation 312 from the source 306. Side 302B is a continuous bottom part of the second collector 302A. The second collector 302 may have an aspherical shape,. e.g., elliptic.

Light from both hemispheres 320 and 330 around the source 306 may be focused with a single reflection by the collectors C1, C2 309, 302 towards an output point or x-z plane 308 with a “z” axis into the page and an “x” horizontal axis. The collectors 302, 309 may be shaped, sized and positioned to reflect light from the source 306 to the output point or plane 308. Examples of values for FIG. 3 may be expressed as follows. For the first collector (C1) 309:

NA₁ (Numerical Aperture)=0.75, L₁=100 mm,

theta₁=sin⁻¹ (NA₁)→H₁=113.389 mm.

1/L₁+1/(a+c)=2/r₁

where “a” and “c” are variables to solve for an elliptic collector in FIGS. 3 and 4. “a” is a major axis of the ellipse 400 in FIG. 4. “b” is a minor axis of the ellipse 400. “c” is a focal point of the ellipse 400 from the origin. r₁ is the radius of curvature of the aspherical collector C1 309. r₂ is the radius of curvature of the ellipse 400 at point “a” in FIG. 4. If L₁, “a” and “c” are known, then the equation above may be solved for r₁.

For the second collector (C2) 302:

(2c−L₄)tan[sin⁻¹ (NA₂)]=

(b/a) (sqr rt of (a−c+L₄)) (sqr rt of (2a−a+c−L₄))

c²=a²b² and

2(sqr rt of (c²+b²))=

(sqr rt of (H₄ ²+(2c−L₄)²))+(sqr rt of (H₄ ²+L₄))

with L₄=400, NA₂=0.25/4=0.0625,

theta₄→H₄

FIG. 4 illustrates a table of values for theta₄ and a, b, c, c/a, r₂, f₁=a−c and r₁. FIG. 4 also illustrates equations for an ellipse 400:

z=a(1−sqr rt of (1−x²/b²))

x=(b/a)(sqr rt of z)(sqr rt of (2a−z))

z=(x²/r₂)/(1+sqr rt of (1−(1+k) (x/r₂)²))

FIG. 5 is a cross-sectional side view of another embodiment of a dual hemispherical collector system 500, which may be used in a lithography illumination system. In FIG. 5, a light source 506 generates light 510 backward in a first hemisphere 530 to a first collector (C1) 509 (also called “surface,” “mirror” or “condenser”) and generates light 512 forward to a set of second collectors (C2) 502, 504, 506.

The first collector 509 may have a spherical or aspherical shape, i.e., elliptic or other conics as well. The first collector 509 focuses light 510 back through a region around the source 506, and the light undergoes reflections from the set of second collectors (C2) 502, 504, 506. Thus, light 510 from the backward direction undergoes reflections from a near normal incidence collector 509 and then grazing incidence collectors 502, 504, 506.

For the first collector (C1) 509:

1/L₁+1/L₂=2/r₁→r=L₁

FIG. 5 also shows the cross-sectional tops 502A, 504A, 506A and bottoms 502B, 504B, 506B of the set of mirrors or grazing incidence collectors 502, 504, 506. The collectors 502, 504, 506 may form a rotation about the optic axis 516. From point 508 looking back at the source 506, the collectors 502, 504, 506 may appear to be three concentric rings (having the same center). The collectors 502, 504, 506 may be symmetric. The collectors 502, 504, 506 may be held together by one or more spokes 514.

Each collector 502, 504, 506 may be a “Wolter” collector, which may be known to those of ordinary skill in the art. For example, the collector 502 may have a first part 503B closer to the source 506 that is generally hyperbolic in shape and a second part 503A farther from the source 506 that is generally elliptic in shape. The collectors 502, 504, 506 collect and reflect forward directed radiation 512 to a point or plane 508.

A Wolter collector includes two conic mirror segments 503A, 503B used to form a grazing incidence imaging optic. The total reflectance loss for small and similar angles of incidence on the two mirrors may be approximately equivalent to the loss from a single larger reflection angle design.

FIG. 6 illustrates an example of a hyperbola 602 and ellipse 600 for modeling a shape of the Wolter collector 502. The hyperbola 602 may have a real focus (focal point) at f1 and a virtual focus at f2. The source 506 in FIG. 5 may be positioned at the real focus f1 of the hyperbola 602. The ellipse 600 may have a first focus at f2 and a second focus at f3. Thus, the first focus of the ellipse 600 is at the virtual or second focus f2 of the hyperbola 602. The output point or image plane 508 may be positioned at f3, which is the second focus f3 of the ellipse 600.

A light beam 604 from the source 506 reflects off the collector 502A to the point/plane 508. Another light beam 606 reflects off the hyperbolic part 503B and then the elliptic part 503A of the collector 502A to the point/plane 508.

The collector systems 300, 500 collect light in a large solid angle from quasi point sources 306, 506, such as a laser produced plasma source used in extreme ultra-violet (EUV) lithography. Common key aspects of the systems 300, 500 may include:

1. A collector system 300, 500 composed of reflective surfaces 302, 304, 309 and 502, 504, 509 in both forward and backward hemispheres 320, 330 and 520, 530 around an object or light source 306 and 506.

2. Reflective surfaces 302, 304, 309 and 502, 504, 509 of the systems 300, 500 may be arranged to project light from the sources 306, 506 onto a point or a plane 308, 508 along the x-z axes.

The systems 300, 500 may have light collection subtending a solid angle greater then 2Π in lithography illumination. Each mirror may be configured and coated separately.

Using both hemispheres 320, 330, 520, 530 may achieve a larger collection angle and more collected radiation.

FIG. 7 illustrates a method of using a dual hemispherical collector system. The method includes providing a first reflective surface in a first hemisphere of a radiation source and a second reflective surface in a second hemisphere of the radiation source at 700; reflecting radiation from the source in the first hemisphere to the second hemisphere at 702; and reflecting radiation from the source in the second hemisphere to a pre-determined plane at 704.

A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the application. Accordingly, other embodiments are within the scope of the following claims. 

1. A lithography system, comprising: a source to emit radiation; a first reflector located at a first location from the source to collect and reflect a first portion of radiation from the source to a target plane, wherein the source is located between the target plane and the first reflector; and a plurality of second reflectors located between the target plane and the target plane and offset from an optical axis between the target plane and the source to collect and reflect at least a second portion of radiation from the source to the target plane, each second reflector comprising a first reflector portion and a second reflector portion in different reflector geometries.
 2. The system as in claim 1, wherein each second reflector is a Wolter collector.
 3. The system as in claim 1, wherein the plurality of second reflectors are symmetrically arranged around the optic axis between the target plane and the LLP source.
 4. The system as in claim 1, wherein the source is a laser produced plasma source. 